Steel sheet plated by hot dipping with alloyed zinc with excellent adhesion and process for producing the same

ABSTRACT

The present invention provides a galvannealed steel sheet excellent in the adhesion with a base steel sheet and a manufacturing method thereof. The galvannealed steel sheet according to the invention has, in an interface between a galvannealed layer and the base steel sheet thereon the galvannealed layer is formed, an irregularity that has a depth of 10 nm or more at a pitch of 0.5 μm or less at least one per 5 μm of a length of the interface.

TECHNICAL FIELD

The present invention relates to a galvannealed steel sheet excellent inthe coating adhesion to a base steel sheet (mother material) and amethod of manufacturing the same.

BACKGROUND ART

In recent years, in the fields of automobiles, home electric appliancesand construction materials, steel sheets that are surface treated toimpart the rust resistance to base steel sheets are used. Among these,galvannealed steel sheets that can be cheaply manufactured and areexcellent in the rust resistance after coating are in use. In the fieldof automobiles in particular, higher mechanical strength and lighterweight of the base steel sheets are in progress. There is an increasingtendency in the use of more galvannealed steel sheets that are rustresistant and high in the mechanical strength.

However, since an interface between a coating layer and a base steelsheet of a galvannealed steel sheet is brittle, for instance, when it ispress-molded with a die, the coating layer peels, and the peeled coatinglayer sticks to the die to deteriorate product quality; accordingly,frequent cleaning of the die is necessary. In some cases, at a portionadhered with a secondary material, the coating layer peels and desiredadhesive strength cannot be obtained. Alternatively, there is a problemin that when an automobile is running in winter, a coating layer comesto peel owing to chipping due to splattered stones or the like, andthereby desired rust resistance cannot be maintained.

In general, a galvannealed steel sheet, after a surface of a base steelsheet is degreased and/or acid washed to cleanse in a pretreatmentprocess or, without applying the pretreatment, an oil content on asurface of the base steel sheet is burned and removed in a pre-heatingfurnace, is preheated in a weak acidic or reducing atmosphere, andundergoes a recrystallization annealing process in a reducingatmosphere. Thereafter, the base steel sheet is cooled in a reducingatmosphere to a temperature suitable for the coating, without exposingto air, dipped in a molten zinc coating bath in which a slight amount ofAl (substantially 0.1 to 0.2 mass percent) is added, followed bycontrolling a coating thickness, and thereby a galvannealed steel sheetis manufactured.

A coating layer of the galvannealed steel sheet is made of an Fe—Znalloy phase that is formed through mutual diffusion of Fe and Zn. In theneighborhood of an interface between the coating layer and the basesteel sheet, an Fe—Zn alloy phase rich in a content of Fe is formed,and, as coming closer toward a surface of the coating layer, an Fe—Znalloy phase poor in the content of Fe is formed. Since the Fe—Zn alloyphase that is formed in the neighborhood of an interface between thecoating layer and the base steel sheet and rich in the content of Fe(for instance, Γ phase and Γ1 phase) is hard and brittle, when it isformed excessively thicker, the brittleness at the interface between thecoating layer and the base steel sheet is enhanced. Furthermore, becausethe coating layer of the galvannealed steel sheet is made of an Fe—Znalloy phase, there is a disadvantage in that since the adhesion of thecoating layer at the interface between the coating layer and the basesteel sheet is poor, peeling at the interface between the coating layerand the base steel sheet is likely to occur.

So far, in the galvannealed steel sheets, a method of improving thecoating adhesion with the base steel sheet has been variously studied.For instance, in Patent Document 1, a technique in which in the case ofultra low carbon IF steel (Interstitial Free Steel) that contains 0.006mass percent or less of carbon being used as a mother material, when Si,P and so on are properly added to steel, Zn in the coating layer ispromoted to diffuse into a grain boundary of the mother material, andthereby the coating adhesion is improved is disclosed. However, inrecent demands for higher mechanical strength, the ultra low carbon IFsteel, being low in the mechanical strength, cannot attain satisfyingperformance. Furthermore, there is a problem in that in the case of ahigh strength steel sheet (for instance, a steel sheet in which carbonand other alloying elements are contained much in a mother material,thereby the tensile strength is made 440 MPa or more) being used, thetechnique according to the Patent Document 1 cannot necessarily obtainsatisfying adhesiveness of the coating layer.

In Patent Document 2, it is disclosed that in the case of P-added steelin which 0.010 to 0.10 mass percent of P and 0.05 to 0.20 mass percentof Si are added to a mother material and Si≧P is satisfied being used,the adhesion of the coating can be improved. However, in the case of thetechnique being applied to steel sheets other than the P-added steelsheet, there is a problem in that satisfying adhesion of the coatinglayer cannot be necessarily obtained.

Furthermore, in Patent Document 3, a technique in which in the case ofhigh strength retained austenite steel in which low carbon steelcontaining 0.05 to 0.25 mass percent of carbon is used as a mothermaterial and proper amounts of Si and Al are added, when proper amountsof Ti, Nb and so on are added in the steel to. fix interstitial C, thecoating interface strength can be improved is disclosed. However, thisis a technique of the retained austenite steel, and there is a problemin that according to the technique described in Patent Document 3, inother high strength steel sheets that do not have a retained austenitephase, sufficient performance cannot be necessarily obtained.

Still furthermore, so far, as to a technique of improving the adhesionof an interface between a coating layer and a steel sheet of agalvannealed steel sheet, various studies have been conducted whilepaying attention to a shape of an interface between the coating layerand the base steel sheet. For instance, in Patent Documents 4 and 5, atechnique in which the surface roughness of a surface of a steel sheetafter a coating layer is removed therefrom is made 6.5 μm or more interms of ten point height of irregularities Rz is disclosed.Furthermore, in Patent Document 6, a technique in which of P-addedsteel, the surface roughness Rz of a surface of the steel after acoating layer is removed therefrom is made to satisfy12≧Rz≧0.0075·Sm+6.7 (where, Rz (μm): ten point height of irregularities,and Sm (μm): average distance between irregularities) is disclosed.However, the present inventors, after studying hard, found a new findingin that in a shape of an interface between the coating layer and thebase steel sheet that contributes to the coating adhesion, fineirregularities that cannot be defined with the ten point height ofirregularities Rz that is used in the existing finding are important,and thereby a galvannealed steel sheet very excellent in the coatingadhesion to an extent that has not been so far found can be obtained.

Patent Document 1: Japanese Patent No. 3163986

Patent Document 2: Japanese Patent No. 2993404

Patent Document 3: JP-A-2001-335908

Patent Document 4: Japanese Patent No. 2638400

Patent Document 5: Japanese Patent No. 2932850

Patent Document 6: Japanese Patent No. 2976845

DISCLOSURE OF INVENTION

The present invention intends to provide a galvannealed steel sheet thatis remarkably excellent in the coating adhesion in comparison with anexisting product, and a manufacturing method thereof.

A gist of the invention is as follows.

(I) A galvannealed steel sheet excellent in the coating adhesion,characterized in that in an interface between a galvannealed layer and abase steel sheet thereon the galvannealed layer is formed, anirregularity having a depth of 10 nm or more at a pitch of 0.5 μm orless is present at least one per 5 μm of a length of an interface.

(II) A galvannealed steel sheet excellent in the coating adhesion,characterized in that, as to a shape of a surface of a base steel sheetthat is observed by peeling a galvannealed layer, a developedinterfacial area ratio Sdr measured by use of a high-pass filter with acut-off wavelength of 0.5 μm is 2.0 percent or more.

(III) The galvannealed steel sheet excellent in the coating adhesionaccording to the (I) or (II), characterized in that the base steel sheetcontains, by mass percent, 0.25 percent or less of C, 0.03 to 2.0percent of Si and 0.005 to 0.07 percent of P and has a compositionsatisfying the following equation (1).

Note[C]+[P]≦[Si]  (1)

Here, [C], [P] and [Si], respectively, mean contents (mass percent) ofC, P and Si in the base steel sheet. (IV) The galvannealed steel sheetexcellent in the coating adhesion according to the (III), characterizedin that in a stage immediately before a coating layer is adhered to thebase steel sheet, in order that Si contained in the base steel sheet maynot be selectively oxidized on a surface, the base steel sheet is heattreated before the coating layer is adhered. (V) The galvannealed steelsheet excellent in the coating adhesion according to the (III) or (IV),characterized in that in base steel immediate below the interface anoxide of silicon is contained.

(VI) The galvannealed steel sheet excellent in the coating adhesionaccording to the (III), (IV) or (V), characterized in that the basesteel sheet has a composition that further includes 5 percent or less ofMn, 0.01 percent or less of S and 0.08 percent or less of Al, by masspercent.

(VII) The galvannealed steel sheet excellent in the coating adhesionaccording to any one of the (III) through (VI), characterized in thatthe base steel sheet has a composition that further includes at leastone kind selected from, by mass percent, 0.2 percent or less of Ti, 0.2percent or less of Nb and 0.2 percent or less of V.

(VIII) A method of manufacturing a galvannealed steel sheet excellent inthe coating adhesion, characterized in that a base steel sheet thatcontains, by mass percent, 0.25 percent or less of C, 0.03 to 2.0percent of Si and 0.005 to 0.07 percent of P and has a compositionsatisfying the following equation (1) is heat treated so that Si in thesteel may not be selectively surface oxidized, followed by cooling to acoating temperature in an atmosphere having an oxygen concentration of0.005 volume percent or less, further followed by dipping the base steelsheet in a molten zinc coating bath to form a coating layer, stillfurther followed by heating at a temperature rise speed of 20 degreecentigrade/s or more to a temperature range of 460 to 600 degreecentigrade and holding in the heating temperature range to apply agalvannealing process of the coating layer.

Note[C]+[P]≦[Si]  (1)

Here, [C], [P] and [Si], respectively, mean contents (mass percent) ofC, P and Si in the base steel sheet. (IX) The method of manufacturing agalvannealed steel sheet excellent in the coating adhesion according tothe (VIII), characterized in that the base steel sheet has a compositionthat further includes, by mass percent, 5 percent or less of Mn, 0.01percent or less of S, and 0.08 percent or less of Al. (X) The method ofmanufacturing a galvannealed steel sheet excellent in the coatingadhesion according to the (VIII) or (IX), characterized in that the basesteel sheet has a composition that further includes at least one kindselected from, by mass percent, 0.2 percent or less of Ti, 0.2 percentor less of Nb and 0.2 percent or less of V and the temperature risespeed and a content of Si in the base steel sheet satisfy the followingequation (2).

NoteST≧3.25/[Si]  (2)

Here, in the equation ST designates a temperature rise speed at agalvannealing process (degree centigrade/s) and [Si] designates acontent (mass percent) of Si in the steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM photograph, in a galvannealed steel sheet according tothe present invention, of a surface of a steel sheet after a coatinglayer is dissolved and removed.

FIG. 2 is a cross sectional SEM photograph of the galvannealed steelsheet according to the invention.

FIG. 3 is a diagram for explaining fine irregularities formed at aninterface between a coating layer and a steel sheet in a galvannealedsteel sheet according to the present invention.

FIG. 4 is a graph showing relationship between a ratio with which fineirregularities formed at an interface between the coating layer and thesteel sheet occupy and the strength at the coating Steel interface.

FIG. 5 is a graph showing relationship between the developed interfacialarea ratio Sdr and the strength of the coating Steel interface.

FIG. 6 is a graph showing, of a steel sheet containing at least one kindof Ti, Nb and V, an influence of a content of Si and a temperature risespeed at a galvannealing process on an area ratio of fineirregularities.

FIG. 7 is a diagram schematically showing a test sample that is used intensile test for evaluating the coating adhesion 1.

FIG. 8 is a diagram schematically showing a test (bending-unbendingtest) for evaluating the coating adhesion 2.

FIG. 9 is a diagram schematically showing a test in which for evaluatingthe coating adhesion 4, a test sample is disposed in a bead die followedby pressing in a horseshoe shape.

FIGS. 10A and 10B each are a 3-D SEM image of a surface of the baseSteel after the coating layer of the galvannealed steel sheet isremoved, FIG. 10A showing a case of a material poor in the adhesion(comparative example), FIG. 10B showing a case of a material excellentin the adhesion (inventive example)

Descriptions of reference numerals in the respective drawings are asfollows.

-   -   1: irregularity curve    -   2: base    -   3, 4: top    -   5: test sample    -   6: adhesive    -   7: spacer    -   8: arrow mark    -   9: test sample    -   10: recessed die    -   11: projected die    -   12: arrow mark    -   13: test sample    -   14: die    -   15: wrinkle suppressor    -   16: bead die    -   17: punch

BEST MODE FOR CARRYING OUT THE INVENTION

In what follows, the present invention will be detailed.

A first invention relates to a galvannealed steel sheet excellent in thecoating adhesion characterized in that in an interface between agalvannealed layer and a base steel sheet thereon the galvannealed layeris formed, an irregularity that has a depth of 10 nm or more at a pitchof 0.5 μm or less is present at least one per 5 μm of a length of theinterface.

The present inventors, after an extensive study, found that when acontinuous fine irregular portion is formed at an interface between acoating layer and a steel sheet, owing to an anchor effect thereof, theadhesion of an interface between the coating layer and the base steelsheet can be remarkably improved.

Each of FIGS. 1 and 2 is a SEM photograph that is taken when acontinuous fine irregular portion at an interface between a coatinglayer and a base steel sheet that is one example of the invention isobserved with a scanning electron microscope (SEM) FIG. 1 is a surfaceSEM photograph observed with a scanning electron microscope when agalvannealed layer is dissolved by applying ultrasonic in an alkalineaqueous solution to be removed and a surface of the base steel sheet atan interface between the coating layer and the base steel sheet isexposed. FIG. 2 is a sectional SEM photograph observed with a scanningelectron microscope after a section of a galvannealed steel sheet ispolished followed by etching with a 0.1 mass percent nital solution. Inthe irregular portion, the finer a pitch is, the more preferable, andthe deeper a depth thereof is, the more preferable. The presentinventors, as a result of study of relationship between the coatingadhesion and the irregular state at the coating interface, found that anabundance of the irregularities that have a depth of 10 nm or more andexist with a pitch of 0.5 μm or less greatly correlates with theadhesive strength of the coating layer. In the irregular portion at aninterface between the coating layer and the base steel sheet, byobserving a section of the coating layer with a scanning electronmicroscope (SEM) or a transmission electron microscope (TEM), a pitchand a depth can be measured. A measuring method thereof will be shownbelow.

Measurements of the pitch and the depth are carried out as follows. Thatis, as shown in FIG. 3, with an irregular curve 1 that is at aninterface and can be confirmed by the section observation, in theirregular curve 1, within a certain reference length L (for instance,0.5 μm), a base 2 that is at a position lowest in height and two tops 3,4 that are at positions highest in height on each of both sides of thebase 2 are found out, a distance in a straight line measured in a lengthdirection between these two tops 3, 4 is taken as a pitch P and adistance in a straight line measured in a height direction between thetop 3 which is the lower one of the two tops 3, 4 and the base 2 istaken as a depth D. When with this measurement method a depth D is 10 nmor more in the reference length L (for instance, 0.5 μm), there is afine irregularity that has a depth D of 10 nm or more at a pitch P of0.5 μm or less.

However, in the invention, it is necessary that the irregularity havinga depth of 10 nm or more at a pitch of 0.5 μm or less exists at leastone per 5 μm of a length of interface. (Here, the length of interfacemeans a distance in a straight line between two points on an interfacein a cross section in a thickness direction.) This is because unless theirregularity exists at this ratio, it does not contribute to animprovement in the coating adhesion. The measurement of theirregularities is carried out as explained below. That is, a crosssection of the coating layer having a length of 10 μm is divided into 20of the reference length L (0.5 μm), 20 viewing fields are observed (Eachof the viewing fields is measured at a magnification of at least 5000times or more.), and, among these, the number of the viewing fields thathave the fine irregularity having a depth D of 10 nm or more at a pitchP of 0.5 μm or less is counted. The measurement is repeated 5 times ofan arbitrary cross section of the coating layer, and a percentage of thenumber of the viewing fields that have the fine irregularity to a totalnumber of viewing fields (20×5=100) is taken as a ratio that the fineirregularities occupy. When the ratio is 10 percent or more, the abovecondition is considered satisfied.

In FIG. 4, relationship between thus measured ratio that the fineirregularities occupy and the strength at the coating Steel interface isshown. From FIG. 4, it is found that when the ratio that the fineirregularities occupy is 10 percent or more, the strength at the coatingsteel interface shows a high value. Here, the strength at the coatingsteel interface is a value obtained by carrying out a tensile testaccording to a method described in a later example (evaluation of thecoating adhesion 1) followed by dividing the tensile strength by anadhered area.

From the above, in the invention, it is necessary that, in an interfacebetween a galvannealed layer and a base steel sheet, an irregularitythat has a depth of 10 nm or more at a pitch of 0.5 μm or less exists atleast one per 5 μm of a length of the interface.

There is the directionality in the formation of the irregularities asshown in FIG. 1. However, a cross section in a direction where theirregularities exist most densely has only to satisfy the condition.

In the next place, a second invention will be explained.

The second invention relates to a galvannealed steel sheet excellent inthe coating adhesion characterized in that, as to a surface shape of abase steel sheet observed after a galvannealed layer is removed, adeveloped interfacial area ratio Sdr measured by use of a high-passfilter with a cut-off wavelength of 0.5 pn is 2.0 percent or more.

The inventors paid attention to a developed interfacial area ratio Sdras an index that can measure from a surface an extent of a continuousirregularity of interfaces of steel sheets shown in FIGS. 1 and 2. Thedeveloped interfacial area ratio expresses a ratio of an area of anactually irregular surface to an area where the irregularity does notexist in a measurement region and is a value expressed by the followingequation.Developed interfacial area ratio (Sdr)=(A−B)/B×100 (%)

-   -   A: a surface area of an actually irregular interface in a        measurement region    -   B: an area of a plane where an irregularity does not exist in a        measurement region

Accordingly, in an interface where the irregularity is large and asurface area is large, the Sdr takes a large value. A shape of thecoating interface of the invention is formed of very fineirregularities; accordingly, quantitative evaluation was very difficult.However, it is considered to evaluate the fine irregularity byexcellently exposing an interface followed by taking a SEM photograph ata high magnification, and thereby precisely calculating the evaluationindex. That is, a surface of a base steel after a coating layer of agalvannealed steel sheetwas removed, after coating with several tensnanometers of Au so as not to affect on a surface composition, wasmeasured with an electron beam three-dimensional surface roughnessanalyzer ERA-8800FE manufactured by Elionics Co., Ltd. followed by shapeanalysis, and thereby the developed interfacial area ratio Sdr wasobtained. The shape analysis was carried out at an accelerating voltageof 15 kV, a viewing field that was magnified at a magnification of 10000(viewing field area is 12 μm×9 μm) was taken in at a resolving power of1200×900 points, followed by data processing. A value of the developedinterfacial area ratio Sdr is obtained by measuring an arbitrarilyselected area followed by averaging. In the calibration that wasperformed in a height direction with the device, a SHS thin film stepstandard (with three steps of 18, 88 and 450 nm) for contact stylus andoptical surface roughness analyzer manufactured by VLSI Standard Inc.having traceable performance to the National Institute of Standards andTechnology in the U.S. was used. Furthermore, a high pass filter havinga cut-off wavelength of 0.5 μm was applied and an obtained value wassupplied for calculation of three-dimensional shape parameter. Theprocessing is important to remove an influence of undulation having along period and thereby to evaluate the irregularities having targetedsizes. The cut-off wavelength as well has to be properly selected to asize of the irregularity that is to be evaluated. After studyingvariously, results processed with a high pass filter having a cut-offwavelength of 0.5 μm were found excellent in the correlation with theinterface strength and in the reproducibility. Accordingly, under thiscondition, the data processing was carried out. Examples of measurementare shown in FIGS. 10A and 10B. FIG. 10A is a 3D-SEM image of a samplepoor in the adhesion (comparative example) and FIG. 10B is a 3D-SEMimage of a product excellent in the adhesion (inventive example), andvalues of the developed interfacial area ratio Sdr, respectively, were1.7 percent for the comparative example and 2.5 percent for theinventive example. That is, there are distinct differences in the imagesand the Sdr values. On the other hand, the Ra in each of the images is0.00531 μm for the comparative example and 0.00547 μm for the inventiveexample. That is, it is found that according to the Ra that is generallyused, the difference cannot be quantified and the effectiveness of theevaluation method can be confirmed.

FIG. 5 is a graph showing relationship between values of the developedinterfacial area ratio Sdr and the strengths of the coating interface atthe interface between the coating layer and the base steel sheet. FromFIG. 5, it is found that in the case of the value of the developedinterfacial area ratio Sdr being 2.0 percent or more, high interfacestrength can be obtained. In the invention, a shape is specified withthe developed interfacial area ratio of three-dimensional parameterconsidered most fitted to the evaluation. However, after processing witha similar high pass filter, it can be evaluated with RSm (an averagelength of roughness curve element) of two-dimensional parameter.

In the next place, a steel sheet suitably used as a base steel sheet ofthe invention will be explained.

A base steel sheet preferably contains, by mass percent, 0.25 percent orless of C, 0.03 to 2.0 percent of Si and 0.005 to 0.07 percent of P andhas a composition satisfying the following equation (1).

Note[C]+[P]≦[Si]  (1)

Here, [C], [P] and [Si], respectively, mean contents (mass percent) ofC, P and Si in the base steel sheet.

Reasons for components C, P and Si in the base steel sheet (mothermaterial) being preferably in the above ranges are as follows. In whatfollows, contents (percent) of elements all mean mass percent.

C: 0.25 Percent or Less

Since the strength of steel can be easily increased when a content of Cis increased, it is indispensable element for increasing the strength ofthe base steel sheet (mother material). However, since when the contentof C is excessive, the ductility or the weldability of the base materialis deteriorated, a content of C is preferably set at 0.25 percent orless. Furthermore, in the case of a steel sheet being used for the deepdrawing, C desirably is not added as far as possible.

Si: 0.03 to 2.0 Percent

Si is a strengthening element of steel and an element that allowsforming a continuous irregular portion at an interface between a coatinglayer and a base steel sheet. Though a detail is not understood, when acontent of Si is less than 0.03 percent, a continuous irregular portionis formed with difficulty. On the other hand, since Si delays analloying reaction, it is preferable not to add as far as possible from aviewpoint of alloying. Furthermore, when a content of Si exceeds 2.0percent, an effect of improving the coating adhesion saturates, and aproblem in that the alloying reaction is excessively delayed is likelyto be caused. Accordingly, a content of Si is preferably in the range of0.03 to 2.0 percent.

P: 0.005 to 0.07 Percent

P is a strengthening element of steel. However, it is a remarkable grainboundary segregation element, delays the reaction excessively anddeteriorates the weldability. Accordingly, it is preferably reduced asfar as possible; that is, P is contained preferably 0.07 percent orless. However, in order to reduce a content of P in the steel more thannecessary, electrolytic iron high in the purity and grade is necessarilyused, resulting in a problem in that economical efficiency is damaged.Accordingly, a content of P is preferably 0.005 percent or more.

In the invention, the contents of C, Si and P in the base steel sheetare limited in the above ranges and preferably satisfy the followingequation (1).

Note[C]+[P]≦[Si]  (1)

-   -   Here, [C], [P] and [Si], respectively, mean contents (mass        percent) of C, P and Si in the base steel sheet.

As mentioned above, when Si is added to steel, a continuous irregularportion is formed at an interface between the coating layer and the basesteel sheet and thereby the coating adhesion can be greatly improved.However, when, in addition to Si, C and P are added in combination inthe steel, a continuous irregular portion is suppressed from forming atan interface between the coating layer and the base steel sheet andthereby an improvement in the coating adhesion is disturbed. Asmentioned above, C and P are strengthening elements of steel andindispensable elements for strengthening. That is, in order to form acontinuous irregular portion that contributes to the coating adhesion,in accordance with amounts of C and P added, an amount of Si added isnecessary to be controlled as shown in the above equation (1). In thecase of [C]+[P]≦[Si], a continuous irregular portion can be easilyformed at an interface between the coating layer and the base steelsheet.

Furthermore, elements other than C, Si and P may be contained in thesteel.

As the other elements, Mn, S and Al can be cited as components that arecontained in the base steel sheet. Preferable ranges of the elements areas follows.

Mn: 5 Percent or Less

Mn is a strengthening element of steel and can be contained as needsarise. However, when a content of Mn exceeds 5 percent, the workabilityand the economic efficiency of the base material are damaged;accordingly, a content Mn is preferably set at 5 percent or less. Inorder to obtain sufficient strengthening effect of the steel, Mn ispreferably contained 0.5 percent or more.

S: 0.01 Percent or Less

S is an element inevitably present in steel. When S is contained morethan 0.01 percent, the workability of the base steel sheet tends todeteriorate. Accordingly, a content of S is preferably set at 0.01percent or less.

Al: 0.08 Percent or Less

Al works as a deoxidizing agent and can be added as needs arise.However, when a content of Al exceeds 0.08 percent, its effect onlysaturates and an increase in the manufacturing cost is invited;accordingly, a content of Al is preferably set at 0.08 percent or less.In order to develop a function as the deoxidizing agent, a content of Alis preferably set at 0.02 percent or more.

Furthermore, as the strengthening element of the steel, at least onekind selected from Ti, Nb and V can be contained. All of Ti, Nb and Ccan bind with C and N in the steel to form a fine precipitate andthereby strengthening the base steel sheet. When each of Ti, Nb and Vcomponents is added more than 0.2 percent, there is a tendency ofdamaging the workability; accordingly, contents of Ti, Nb and V each arepreferably set at 0.2 percent or less.

Furthermore, at least one kind selected from Ti, Nb and V, when added ina proper amount, combines with dissolved P to form a fine precipitate,Fe—(Ti, Nb, V)—P, and thereby the dissolved P is partly renderedharmless. As a result, without excessively delaying a mutual diffusionreaction of Fe and Zn, the coating interface strength can be largelyimproved. In order to develop such an effect, in accordance with anamount of P in the steel, at least one kind of Ti, Nb and V satisfyingthe following equation (3) is preferably contained.[Ti]+[Nb]+[V]≧[P]  (3)

-   -   Here, [Ti], [Nb], [V] and [P], respectively, mean contents (mass        percent) of Ti, Nb, V and P.

Components such as Cr, Mo, Cu, Ni, Ca, B, N and Sb other than theabovementioned components in the base steel sheet, since presencethereof does not at all contribute to the effects of the invention, maybe added as needs arise. Reasons for addition and preferable rangesthereof are as follows.

Cr: 0.5 Percent or Less

This is a strengthening element of steel and can be added as needsarise. However, since the coating properties are deteriorated and thealloying nonuniformity is caused, it is preferably added by 0.5 percentor less.

Mo: 1.0 Percent or Less

This is a strengthening element of steel and can be added as needsarise. However, since the alloying delay is caused and the workabilityand the economic efficiency are damaged, it is preferably added by 1percent or less.

Cu: 0.5 Percent or Less

This is a coating property improving element and can be added as needsarise. However, when it is added more than 0.5 percent, an effectthereof saturates and the economic efficiency is damaged. Accordingly,it is preferably added by 0.5 percent or less.

Ni: 0.5 Percent or Less

This is a coating property improving element and can be added as needsarise. However, when it is added more than 0.5 percent, an effectthereof saturates and the economic efficiency is damaged. Accordingly,it is preferably added by 0.5 percent or less.

Ca: 0.01 Percent or Less

This works as a deoxidizing agent and may be contained as needs arise.However, when it is added more than 0.01 percent, an effect thereofsaturates. Accordingly, an addition of 0.01 percent or less ispreferable.

B: 0.003 Percent or Less

Owing to grain boundary strengthening, the cold work embrittlement canbe improved. However, since an effect thereof saturates at more than0.003 percent, it is preferably added by 0.003 percent or less.

N: 0.01 Percent or Less

N comes in as an impurity. When it exceeds 0.01 percent, the ductilityis deteriorated. Accordingly, it is preferably added by 0.01 percent orless.

Sb: 0.05 Percent or Less

This is a coating appearance improvement element and can be added asneeds arise. However, when it is added more than 0.05 percent, an effectthereof saturates and the economic efficiency is damaged. Accordingly,it is preferably added by 0.05 percent or less.

The balance other than the abovementioned elements is preferably made ofFe and inevitable impurities.

Furthermore, in the invention, the tensile strength of the base steelsheet that is measured with a No. 5 test piece stipulated in JIS Z2201and according to a tensile test method stipulated in JIS G3302 ispreferably 440 MPa or more. When the base steel sheet is made a hightension steel sheet having the tensile strength of 440 MPa or more, inthe fields of automobiles, home electric appliances, constructionmaterials and so on, demands for higher strength and/or lighter weightbase can be satisfied.

In the next place, a manufacturing method of forming an irregularityaccording to the invention (an irregularity that has a depth of 10 nm ormore at a pitch of 0.5 μm or less and is present at least one per 5 μmof a length of the interface or an irregularity that has the developedinterfacial area ratio Sdr of 2.0 percent or more when a surface shapeof a base steel sheet observed by peeling a galvannealed layer ismeasured with a high pass filter with a cut-off wavelength of 0.5 μm) atan interface between a galvannealed layer and a base steel sheet will beexplained below.

A galvannealed steel sheet according to the invention can bemanufactured, with a steel sheet having, for instance, theabovementioned component composition as a base steel sheet, by applyinga hot-dip galvanizing process and a subsequent galvannealing process.Here, the base steel sheet may be any one of a hot rolled steel sheet, acold rolled steel sheet, or a steel sheet obtained by speciallyheat-treating these and is not restricted to particular one. The basesteel sheet, after a surface thereof is cleansed by degreasing and/or bywashing with acid in a pre-treatment process, or, by omitting thepre-treatment process, an oil component on a surface of the base steelsheet is burned and removed in a pre-heating furnace, is annealed at atemperature in the range of substantially 750 to 900 degree centigradein a reducing atmosphere. Thereby, a scale on the surface of the basesteel sheet is reduced and a surface state suitable for subsequenthot-dip galvanizing is obtained. Now, in the case of the base steelsheet in which Si is added to steel, even in a reducing atmosphere toFe, in some cases, Si is selectively surface oxidized, resulting informing an oxide concentrated on a surface. The silicon oxide oxidizedselectively on a surface deteriorates the wettability with molten zincduring the coating to result in causing a bare spots surface.Accordingly, it is necessary to suppress the selective surface oxidationin a reducing atmosphere. Furthermore, as mentioned above, although Siin steel has a function of allowing forming a fine irregular portion atan interface between a coating layer and a base steel sheet, sincesilicon does not develop an effect when it exists as oxide, it isnecessary to substantially suppress the selective surface oxidation in areducing atmosphere from occurring.

Substantially suppressing the selective surface oxidation of Si fromoccurring means as mentioned above a state where the coating wettabilityis lowered and thereby the bare spots is inhibited from occurring; thatis, there is no problems as far as it is a state where the bare spots isnot caused.

As a method of obtaining a state where, with steel to which Si is added,Si does not substantially undergo the selective surface oxidation in areducing atmosphere, though not particularly restricted, there is amethod in which, prior to annealing in a reducing atmosphere, in a weakacidic atmosphere, for instance, in an inert gas atmosphere containing aslight amount such as 1 volume percent or less of oxygen, a pre-heatingor heating process is applied. That is, in a weak acidic atmosphere asurface of the steel sheet is oxidized to form a thin iron scalefollowed by annealing in a reducing atmosphere to form reduced iron onthe surface of the steel sheet, and thereby the selective surfaceoxidation of Si can be suppressed from occurring. The weak acidicatmosphere is an acidic atmosphere to an extent that allows sufficientlyapplying reduction in a later reducing atmosphere and not particularlyrestricted. As a weak acidic atmosphere, for instance, an atmospherewhere 0.01 to 0.5 volume percent of oxygen is contained, a dew point isin the range of −20 to +20 degree centigrade, the balance is made ofnitrogen and a temperature is in the range of 300 to 500 degreecentigrade can be cited, and as a reducing atmosphere, for instance, anatmosphere where 3 to 20 volume percent of hydrogen is contained, thebalance is made of nitrogen and a temperature is in the range of 750 to900 degree centigrade can be cited.

When a surface of a steel sheet is oxidized in a weak acidic atmosphereto form a thin iron scale followed by annealing in a reducing atmosphereand thereby reduced iron is formed on a surface of the steel sheet, Feoxide formed in the weak acidic atmosphere is reduced in an annealingprocess in the subsequent reducing atmosphere and silicon oxide, withoutbeing oxidized even in the annealing process in the reducing atmosphere,remains as internal oxide in base steel immediate below a surface of thebase steel sheet. The internal oxide is distinguished from an oxide thatis formed according to the selective surface oxidization of Si and worksso as to suppress Si from being selectively surface oxidized during theannealing in a reducing atmosphere. The internal oxide remains in ahot-dip galvanizing process and in a subsequent galvannealing process.

When the pre-heating or heating process in a weak acidic atmospherecannot be applied from an apparatus point of view, after a primaryheating is applied at a relatively high temperature in the range of 800to 900 degree centigrade in a reducing atmosphere, a surface oxide isremoved by applying pickling or polishing. Subsequently, after asecondary heating is carried out at a relatively low temperature of 800degree centigrade or less in a reducing atmosphere, the coating isapplied without exposing to air, and thereby Si can be substantiallysuppressed from being selectively surface oxidized. A method ofobtaining a state where, as mentioned above, Si is not substantiallyselectively surface oxidized in a reducing atmosphere is not restrictedto particular one and in any method an effect of the invention is notdisturbed.

The base steel sheet after the annealing is cooled in the reducingatmosphere to a temperature suitable for the coating, preferably in therange of 440 to 540 degree centigrade, dipped without exposing to air ina molten zinc coating bath to apply the coating. At this time, anatmosphere immediately before the coating is made an atmosphere havingan oxygen concentration of 0.005 volume percent or less. This is becauseoxygen, in particular, lowers the reactivity of a surface of the basesteel sheet to disturb the formation of a fine irregularity at aninterface between a coating layer and the base steel sheet. Residualgases other than oxygen, not particularly affecting on the formation ofthe fine irregularity, are not limited. For instance, an atmospherecontaining 3 to 20 volume percent of hydrogen and the balance ofnitrogen can be cited. Furthermore, since oxygen lowers the wettabilitywith molten zinc to induce the bare spots, also from this meaning, it isbetter to be low.

The hot-dip galvanizing process has only to be conducted according to anexisting method. For instance, it is preferable that a temperature of acoating bath is set in the range of substantially 450 to 500 degreecentigrade and a concentration of Al in the coating bath is set in therange of 0.10 to 0.15 mass percent. However, depending on components inthe steel, the coating conditions mentioned above have to be altered.However, difference of the coating conditions, not bringing about anycontribution to the effects of the invention, is not particularlyrestricted.

As a method of adjusting a thickness of a coating layer after thecoating, without being restricted to a particular one, a generalgas-wiping is used; that is, a gas pressure of the gas-wiping, adistance between a wiping nozzle and a steel sheet and so on are used toadjust. At this time, a thickness of the coating layer is preferably inthe range of 3 to 15 μm. When it is less than 3 μm, the rust resistancecannot be sufficiently obtained. On the other hand, when it exceeds 15μm, not only an improving effect of the rust resistance saturates butalso the workability and the economic efficiency unfavorably tend to belowered.

A method of galvannealing process after the coating thickness isadjusted can be applied by use of a method such as gas heating orinduction heating. However, it is necessary that an average temperaturerise speed during heating to a galvannealing temperature is 20 degreecentigrade/s or more. This is because in the case of less than 20 degreecentigrade/s, a staying time in a low temperature region is long tocause a delay in galvannealing reaction, and thereby a fine irregularityat an interface between a coating layer and a base steel sheet isinhibited from forming.

Furthermore, in the case of Ti, Nb and V being contained in the aboverange in a base steel sheet, a temperature rise speed during heating inthe galvannealing process and a content of Si in the base steel sheetare necessary to satisfy the equation (2) below.ST≧3.25/[Si]  (2)

Here, in the equation, ST expresses a temperature rise speed (degreecentigrade/s) and [Si] denotes a content (mass percent) of Si in thesteel sheet.

According to inventors' research, it was found that when Ti, Nb and Vare contained in steel, in the case of a content of Si being low, evenwhen a temperature rise speed in the galvannealing process is set at 20degree centigrade/s or more, in some cases, an inventive fineirregularity in an interface between the coating layer and the basesteel sheet is not formed; that is, a temperature rise speed isnecessary to raise in accordance with the content of Si.

FIG. 6 is a graph showing, of steel sheets that contain at least onekind of Ti, Nb and V in a range that satisfies the equation (3),influence of a content of Si and a temperature rise speed on a an arearatio of fine irregularity. It is found that when the equation (2) issatisfied, the area ratio of the fine irregularity becomes 10 percent ormore.

Although a time of galvannealing is not particularly restricted, acontent of Fe in the coating layer is preferably controlled in the rangeof 8 to 13 mass percent. When the content of Fe in the coating layer isless than 8 mass percent, since the aforementioned Fe—Zn alloy phase isnot sufficiently formed and a soft η-Zn phase remains on a surface ofthe coating layer, in some cases, the workability and the adhesion aredamaged. On the other hand, when the content of Fe in the coating layerexceeds 13 mass percent, there is a problem in that a hard and brittleFe—Zn alloy phase (for instance, a Γ phase or a Γ1 phase) is formedexcessively thick in an interface between the coating layer and the basesteel sheet, and thereby the embrittlement in the interface between thecoating layer and the steel sheet is forwarded.

“A content of Fe in a coating layer” here denotes a mass percentage ofFe in a coating layer to an entire coating layer, that is, an averagecontent of Fe. A method of measuring a content of Fe in the coatinglayer is carried out in such manner that for instance, a galvannealedlayer is dissolved with hydrochloric acid added with an inhibitorfollowed by measuring by ICP (Inductively Coupled Plasma) emissionspectrometry.

A method of controlling a content of Fe in the coating layer in therange of 8 to 13 mass percent is not restricted to particular one. Ingeneral, it is controlled through a sheet temperature and a staying timein a galvannealing heating furnace and so on. The staying time in thefurnace is preferably shorter from a viewpoint of the productivity andspecifically operated within substantially 5 to 30 sec. Furthermore, thesheet temperature, though being selected depending on the staying timein the furnace, is generally operated in the range of 460 to 600 degreecentigrade. In the case of less than 460 degree centigrade, in order tocontrol the content of Fe in the coating layer in the range of 8 to 13mass percent, a long galvannealing process is forced to operate;accordingly, it becomes necessary to make a speed of steel sheetextremely slow or to use a very long galvannealing furnace. As a result,since there is a problem in that the productivity is lowered or hugeequipment expense is necessary, it is preferably operated at 460 degreecentigrade or more. On the other hand, when it exceeds 600 degreecentigrade, there is a problem in that in an interface between thecoating layer and the base steel sheet, a hard and brittle Fe—Zn alloyphase (for instance, a Γ phase or a Γ1 phase) tends to be formedexcessively thick, and thereby the embrittlement of the interfacebetween the coating layer and the base steel sheet is enhanced.Accordingly, it is preferably operated at 600 degree centigrade or less.

After the galvannealing process, cooling is immediately followed. Amethod of cooling, though not particularly restricted, is desirablyapplied by quenching at 30 degree centigrade/s or more to 420 degreecentigrade where the galvannealing reaction comes to completion, forinstance, an existing method such as gas cooling and mist cooling hasonly to be applied.

In what was mentioned above, only one example of embodiments of theinvention is shown and the invention can be variously modified in therange of claims.

EXAMPLE 1

Each of steel ingots having a chemical composition shown in Table 1 washeated to 1250 degree centigrade to apply hot rolling followed byremoving a scale on a surface, and thereby a hot rolled steel sheethaving a thickness of 2.0 mm was prepared. Subsequently, cold rolling atthe reduction rate of 50 percent was applied to form a cold rolled steelsheet having a thickness of 1.0 mm, followed by cutting out into a widthof 70 mm and a length of 180 mm. This was subjected to primary heatingat 830 degree centigrade in a heating furnace in a nitrogen atmospherethat contains 3 volume percent of hydrogen and has a dew point of −30degree centigrade to cleanse a surface thereof, and thereby a base steelsheet was prepared. After the base steel sheet was dipped in 5 percenthydrochloric acid at 60 degree centigrade for 10 sec to apply pickling,recrystallization annealing and hot-dip galvanizing (hereinafter, simplyreferred to as “galvanizing”) were applied by use of a laboratorygalvanizing simulator. Conditions for the recrystallization annealingand the galvanizing were as follows. TABLE 1 The balance of steelcomposition (mass %) is Fe Steel and inevitable impurities No. C Si Mn Psol.Al S Note 1A 0.03 0.1 2.2  0.065 0.03 0.003 Example 1B 0.08 0.1 0.50.01 0.029 0.003 1C 0.08  0.25 2 0.01 0.042 0.003 1D 0.08 0.2 2.6  0.0150.035 0.003 1E 0.03 0.6 2 0.01 0.05 0.003 1F 0.08 0.2 2 0.01 0.041 0.0031G 0.08 0.6 1.95 0.01 0.045 0.003 1H 0.15 0.8 2.6  0.012 0.065 0.003 1I0.1  0.25 2  0.015 0.029 0.003 1J 0.03  0.25 1.6 0.03 0.033 0.003 1K0.16 0.2 0.8 0.01 0.041 0.003 1L 0.25 0.3 0.8  0.012 0.041 0.003 1M 0.030.5 1.5 0.02 0.036 0.003 1N 0.003  0.02 0.28 0.02 0.031 0.003Comparative 1O 0.002  0.02 0.09  0.014 0.04 0.003 Example 1P 0.15  0.051.2  0.012 0.039 0.003 1Q 0.15 0.1 1.2  0.012 1.5 0.003 1R 0.05  0.020.8  0.008 0.055 0.003 1S 0.018  0.02 0.18 0.01 0.033 0.003 1T 0.01 0.11  0.075 0.035 0.003 1U 0.004  0.02 0.14  0.021 0.045 0.003 1V 0.08 0.07 2 0.01 0.06 0.003 1W 0.002  0.02 0.3  0.035 0.033 0.003 1X 0.120.1 3  0.015 1.5 0.003 1Y 0.08  0.05 1.5 0.03 0.041 0.003<Recrystallization Annealing>

-   Atmosphere: 5 volume percent hydrogen+nitrogen (dew point: −35    degree centigrade)-   Temperature: 750 degree centigrade-   Holding time: 20 sec    <Coating condition>-   Bath composition: Zn+0.14 mass percent Al (Fe saturation)-   Bath temperature: 460 degree centigrade-   Sheet temperature at the time of coating: 460 degree centigrade-   Coating time: 1 sec-   Concentration of oxygen in an atmosphere immediately before the    coating: conditions described in Table 2 (the balance 5 volume    percent hydrogen+nitrogen (dew point: −35 degree centigrade))

Obtained coating steel sheets contained 0.2 to 0.5 mass percent of Aland 0.5 to 2 mass percent of Fe in the coating layers. After the coatingprocess above, a galvannealing process was applied in air in an electricheater. Temperature rise speeds and galvannealing temperatures in thegalvannealing process were the conditions described in Table 2.

Of each of obtained coating steel sheets, a cooling atmosphere from therecrystallization annealing to the coating, a thickness of a coatinglayer, a temperature rise speed, a temperature and a holding time in thegalvannealing process, a content of Fe in the coating layer, a ratio offine irregularity formed in an interface between the coating layer and abase steel sheet and a developed interfacial area ratio Sdr are shown inTable 2. Furthermore, a method of evaluating the coating adhesion 1 ofthe obtained coating steel sheet is shown below and evaluation resultsare shown together in Table 2.

<Ratio of Interfacial Irregularity>

A cross section of an interface of the coating layer and the steel sheetin the obtained steel sheet was observed with a SEM (TEM was usedtogether) over a length of 10 μm in five viewing fields in an arbitrarycross section and a ratio at which fine irregularity (having a depth of10 nm or more at a pitch of 0.5 μm or less) occupies in an entirecoating cross section is taken as an interfacial irregularity ratio (%).

<Developed Interfacial Area Ratio Sdr>

The coating layer was removed by subjecting to constant-potentialelectrolysis in an alkaline solution containing NaOH, NaCl, andtriethanolamine and thereby an interface between the coating layer andthe base steel sheet was exposed. The exposed surface was measured of asurface shape by use of an electron beam three-dimensional surfaceroughness analyzer ERA-8800FE (manufactured by Elionics Co., Ltd.). Atest sample, in order to avoid an influence of a composition of surface,was coated with Au with a thickness of several tens nanometers andsupplied for measurement. The shape analysis measurement was performedat an acceleration voltage of 15 kV, a viewing field magnified by 10000times (viewing field area is 12 μm×9 μm) was collected at the resolvingpower of 1200×900 points, followed by data processing. A value of thedeveloped interfacial area ratio Sdr was obtained by averaging resultsobtained by measuring arbitrarily selected three areas. In thecalibration that was performed in a height direction with the device, aSHS thin film step standard (with three steps of 18, 88 and 450 nm) forcontact stylus and optical surface roughness analyzer manufactured byVLSI Standard Inc. having traceable performance to the NIST that isNational Institute of Standard and Technology in the U.S. was used.Furthermore, a high pass filter having a cut-off wavelength of 0.5 μmwas applied to supply for calculation of three-dimensional shapeparameter.

<Thickness of Coating Layer>

A cross section of the obtained coating steel sheet was observed with anoptical microscope (magnification: 400 times) a thickness of the coatinglayer was measured at arbitrary three points, followed by averagingthese, and an averaged value was taken as a thickness of the coatinglayer (μm).

<Content of Fe in the Coating Layer>

The coating layer of the obtained coating steel sheet was dissolved withhydrochloric acid added with an inhibitor and Zn and Fe in the coatinglayer were quantitatively analyzed by ICP emission spectrometry. A masspercentage (mass percent) of Fe to (Zn+Fe) was taken as a content of Fein the coating layer.

(Evaluation of the coating adhesion 1)

From the obtained coating steel sheet, two test pieces having a width of25 mm and a length of 80 mm were cut out, after dipping in a rustpreventive oil: 550 KH (manufactured by Nihon Parkerizing Co., Ltd.),were left in air standing obliquely for 24 hr, and thus obtained oneswere supplied as test samples. As shown in FIG. 7, after an adhesive 6was coated on surface portions that are adhered of test samples 5, thetest samples were stacked so that a length of an overlapped portion Xmay be 20 mm. As the adhesive 6, E-56 (manufactured by Sunrise MSI Co.,)was used, and by use of spacers 7 (SUS304 wire having a diameter of 0.15mm) a thickness of the adhesive was maintained constant for each of thetest pieces. After the adhesive was coated, heat treatment was appliedat 170 degree centigrade for 20 min in a drying oven, thereafter tensiletest applying tension in directions of arrow marks 8 was applied by useof an autograph (manufactured by Shimadzu Corporation), and thereby thetensile shear strength and peeling mode were measured, followed byevaluating according to criteria below. The tensile shear strength wasevaluated with a ratio (%) to the strength obtained when with a coldrolled steel sheet (non-coating material) having the same steelcomposition and the same size the tensile test was applied.

<Evaluation Criteria of Tensile Shear Strength>

-   ◯◯: very good (strength ratio: exceeding 90%)-   ◯: good (strength ratio: exceeding 80% and 90% or less)-   Δ: fair (strength ratio: exceeding 60% and 80% or less) and-   x: bad (strength ratio: 60% or less)    <Evaluation Criteria of Peeling Mode>-   ◯◯: very good (coagulation peeling in the adhesive)-   Δ: fair (partially peeling at an interface of coating layer/base    steel sheet)-   x: bad (overall peeling at an interface of coating layer/base steel    sheet)

In the evaluation criteria of the peeling mode, the peeling at aninterface of coating layer/base steel sheet means the peeling at aninterface of the coating layer and the base steel sheet. However,depending on the peeling mode, in some cases, the peeling at aninterface of the coating layer and the base steel sheet does not occuruniformly, accordingly cases where the peeling occurs within 2 μm on aside of the coating layer or on a side of the base steel sheet from theinterface of the coating layer and the base steel sheet are included.TABLE 2 Galvannealed steel sheet Concentration of Galvannealing Basesteel Evaluation oxygen in a cooling condition sheet result atmosphereuntil Temper- Developed Ratio of fine Coating Test the coating afterature Coating layer interfacial irregularity adhesion 1 sam- therecrystallization rise Galvannealing Holding Thick- Content of area inan Tensile ple Steel annealing speed temperature time ness Fe ratio Sdrinterface shear Peeling No. No. (vol. %) (° C./s) (° C.) (s) (μm) (mass%) (%) (%) strength mode Note 1 1A 0.002 25 490 15 7 10.8 2.2 15 ◯ ◯◯Exam- 2 1B 0.002 20 480 10 6 9.2 2.1 15 ◯◯ ◯◯ ple 3 1C 0.001 25 490 9 310.3 2.5 50 ◯◯ ◯◯ 4 1C 0.002 25 490 15 7 9.9 2.5 45 ◯◯ ◯◯ 5 1C 0.002 25490 22 6 12.5 2.8 75 ◯◯ ◯◯ 6 1C 0.003 30 510 20 14 11.2 2.6 60 ◯◯ ◯◯ 71D 0.002 25 500 16 9 11.8 2.6 55 ◯◯ ◯◯ 8 1E 0.002 35 520 20 8 10.6 2.665 ◯◯ ◯◯ 9 1F 0.002 25 490 15 10 11.0 2.3 30 ◯◯ ◯◯ 10 1G 0.002 30 520 156 11.3 2.5 50 ◯◯ ◯◯ 11 1H 0.004 30 520 20 6 10.6 2.3 25 ◯◯ ◯◯ 12 1I0.002 20 460 12 4 9.1 2.1 10 ◯ ◯◯ 13 1J 0.002 25 490 20 7 10.6 2.8 70 ◯◯◯◯ 14 1K 0.002 30 510 15 6 11.2 2.8 75 ◯◯ ◯◯ Exam- 15 1L 0.004 25 480 188 11.0 2.8 65 ◯◯ ◯◯ ple 16 1M 0.002 35 540 6 5 9.2 2.8 70 ◯◯ ◯◯ 17 1N0.003 20 520 8 7 10.0 1.6 0 X Δ Com- 18 1O 0.002 30 470 15 10 9.5 1.5 0X X para- 19 1P 0.002 20 500 20 6 12.3 1.7 0 Δ X tive 20 1Q 0.002 20 49015 6 10.0 1.9 0 Δ Δ Exam- 21 1R 0.002 35 490 7 7 8.2 1.8 0 Δ Δ ple 22 1S0.003 20 520 15 8 12.8 1.4 0 X X 23 1T 0.002 20 520 22 9 11.5 1.7 0 X Δ24 1U 0.002 20 510 12 10 11.5 1.6 0 X X 25 1V 0.001 20 500 9 8 9.9 1.8 0X Δ 26 1W 0.002 30 490 12 10 9.6 1.9 0 Δ Δ 27 1X 0.002 20 520 15 11 11.61.7 0 X X 28 1Y 0.002 20 470 18 9 11.1 1.7 0 Δ Δ 29 1B 0.007 20 480 10 69.2 1.9 5 Δ Δ

From the evaluation results shown in Table 2, it is found thatgalvannealed steel sheets according to the invention (examples), incomparison with existing steel sheets (comparative examples), arelargely heightened in the strength of the interface between the coatinglayer and the base steel sheet and improved in the coating adhesionthereof.

EXAMPLE 2

Each of steel ingots having a chemical composition shown in Table 3 washeated at 1250 degree centigrade to apply the hot rolling followed byremoving a scale on a surface, and thereby a hot rolled steel sheethaving a thickness of 2.0 mm was prepared. Subsequently, the coldrolling at the reduction rate of 50 percent was applied to form a coldrolled steel sheet having a thickness of 1.0 mm, followed by cutting outinto a width of 70 mm and a length of 180 mm, further followed bysurface cleaning, and thereby a base steel sheet was obtained. The basesteel sheet was dipped in 5 percent hydrochloric acid at 60 degreecentigrade for 10 sec to apply pickling, thereafter, subjected toprimary heating by holding at 400 degree centigrade for 1 sec in anitrogen atmosphere (dew point: +20 degree centigrade) containing 0.1volume percent of oxygen, and thereafter subjected to a secondaryheating by holding at 750 degree centigrade for 1 sec in a nitrogenatmosphere (dew point: +20 degree centigrade) containing 5 volumepercent of hydrogen. To the heat treated base steel sheet,recrystallization annealing and coating were applied by use of alaboratory galvanizing simulator. Conditions for the recrystallizationannealing and the coating were as follows. TABLE 3 The balance of steelcomposition (mass %) is Fe and inevitable impurities Steel No. C Si Mn PTi Nb V 3.25/Si Note 2A 0.025 0.13 2 0.03 0.02 0.01 0.01 25 Example 2B0.08 0.1 0.5 0.01 0.02 0.01 — 33 2C 0.08 0.25 2 0.01 0.02 0.06 — 13 2D0.08 0.2 2.6 0.015 0.02 0.05 — 16 2E 0.075 0.6 2 0.01 — 0.03 — 5 2F 0.080.2 2 0.01 0.02 — — 16 2G 0.08 0.6 1.95 0.01 0.01 0.01 — 5 2H 0.15 0.82.6 0.012 0.01 0.01 — 4 2I 0.1 0.3 2 0.015 — 0.02 0.02 11 2J 0.08 0.251.6 0.03 —  0.025 0.05 13 2K 0.16 0.2 0.8 0.01 0.01 0.01 — 16 2L 0.250.3 0.8 0.012 0.02 0.03 — 11 2M 0.04 0.16 3 0.04 0.02 0.03 0.01 20 2N0.003 0.02 0.28 0.02 0.02 0.01 — 163 Comparative 2O 0.002 0.02 0.090.014 0.02 0.01 — 163 Example 2P 0.15 0.1 1.2 0.012 0.01 — — 33 2Q 0.150.02 1.2 0.012 0.02 0.01 0.01 163 2R 0.05 0.02 0.8 0.008 0.02 0.05 — 1632S 0.018 0.02 0.18 0.01 0.02 0.01 — 163 2T 0.01 0.12 1 0.075 0.02 0.05 —27 2U 0.004 0.03 0.14 0.04 0.01 0.01 — 108 2V 0.08 0.07 2 0.01 0.02 0.01— 46 2W 0.002 0.02 0.1 0.01 0.01 0.01 — 163 2X 0.002 0.03 0.3 0.035 0.020.01 0.02 108 2Y 0.12 0.02 1.5 0.015 0.02 0.01 — 163 2Z 0.08 0.05 1.50.03 0.02 0.03 — 65<Recrystallization annealing>

-   Atmosphere: 5 volume percent hydrogen+nitrogen (dew point: −35    degree centigrade)-   Temperature: 830 degree centigrade-   Holding time: 20 sec    <Coating Condition>-   Bath composition: Zn+0.13 mass percent Al (Fe saturation)-   Bath temperature: 460 degree centigrade-   Sheet temperature at the time of coating: 460 degree centigrade-   Coating time: 1 sec-   Concentration of oxygen in an atmosphere immediately before the    coating: conditions described in Table 4 (the balance 5 volume    percent hydrogen+nitrogen (dew point: −35 degree centigrade))

Obtained coating steel sheets contained 0.2 to 0.5 mass percent of Aland 0.5 to 2 mass percent of Fe in the coating layer. After the coatingprocess, the galvannealing process was applied in air in an electricheater. The temperature rise speeds and galvannealing temperatures inthe galvannealing process were the conditions described in Table 4.

Of each of obtained coating steel sheets, a cooling atmosphere from therecrystallization annealing to the coating, a thickness of a coatinglayer, a temperature rise speed, a temperature and a holding time in thegalvannealing process, a content of Fe in the coating layer, a ratio offine irregularity formed in an interface between the coating layer and abase steel sheet and a developed interfacial area ratio Sdr wereinvestigated similarly to a method explained in the example 1.Furthermore, in addition to the evaluation of the abovementioned coatingadhesion 1, evaluation of the coating adhesion 2 shown below was carriedout. Results of these are shown in Table 4. Furthermore, a method ofevaluating the coating adhesion of the obtained coating steel sheet isshown below and evaluation results are shown together in Table 4.

(Evaluation of the Coating Adhesion 2)

From each of the obtained steel sheets, a test piece having a width of20 mm and a length of 180 mm was cutout followed by removing burrs,after dipping in rust-preventive oil 550 KH (manufactured by NihonParkerizing Co., Ltd.), left in air for 24 hr while standing obliquely,and thus obtained one was used as a test sample. A test sample 9 wasdisposed on a recessed die 10 such as shown in FIG. 8, and a test inwhich a bending and unbending operation is applied by lowering aprojected die 11 and thereby indenting a surface of the test sample 9with a load W was carried out. A surface of the die was polished with#1200 polishing paper and cleaning of accretions was carried out eachtime. An indentation load P of the die was set at 8 kN and the drawingspeed of the test sample was set at 20 mm/s. After the test, the testsample was slightly degreased, followed by adhering a cellophane tape(width: 24 mm, manufactured by Nichiban Corp.) to a sliding portion withthe die. An amount of Zn adhered to the cellophane tape when it waspeeled was measured as the number of counts by X-ray fluorescenceanalysis, and evaluation was carried out according to the followingcriteria.

<Evaluation Criteria of the Coating Adhesion 2>

-   ◯◯: Very good (number of counts: 25 or less)-   ◯: good (number of counts: more than 25 and 50 or less)-   Δ: fair (number of counts: more than 50 and 150 or less)

x: bad (number of counts: more than 150) TABLE 4 Concentration of oxygenin a Galvannealed steel sheet cooling Base steel atmosphere Alloyingcondition sheet Ratio of Evaluation result until the Temper- Coatinglayer Developed fine Coating Test coating after the ature AlloyingContent interfacial irregularity adhesion 1 Coating sam-recrystallization rise temper- Holding Thick- of Fe area in an Tensileadhe- ple Steel annealing speed ature time ness (mass ratio Sdrinterface shear Peeling sion No. No. (vol. %) (° C./s) (° C.) (s) (μm)%) (%) (%) strength mode 2 Note 1 2A 0.001 30 520 15 6 10.5 2.6 60 ◯ ◯◯◯◯ Exam- 2 2B 0.002 35 480 12 7 9.5 2.6 50 ◯◯ ◯◯ ◯◯ ple 3 2C 0.001 25490 10 3 10.5 2.6 55 ◯◯ ◯◯ ◯◯ 4 2C 0.001 25 490 15 7 9.9 2.5 50 ◯◯ ◯◯ ◯◯5 2C 0.002 25 490 25 6 12.8 2.8 70 ◯◯ ◯◯ ◯ 6 2C 0.002 25 520 25 14 11.02.7 65 ◯◯ ◯◯ ◯◯ 7 2D 0.002 25 500 15 9 11.6 2.6 50 ◯◯ ◯◯ ◯◯ 8 2E 0.00325 520 17 8 10.4 2.5 50 ◯◯ ◯◯ ◯◯ 9 2F 0.002 25 490 15 11 11.2 2.6 60 ◯◯◯◯ ◯◯ 10 2G 0.002 25 500 20 6 10.9 2.6 50 ◯◯ ◯◯ ◯◯ 11 2H 0.004 25 520 156 9.9 2.5 45 ◯◯ ◯◯ ◯◯ 12 2I 0.002 25 460 8 4 8.9 2.1 15 ◯ ◯◯ ◯◯ 13 2J0.001 25 490 20 7 10.6 2.5 50 ◯◯ ◯◯ ◯◯ 14 2K 0.002 25 460 30 7 11.2 2.660 ◯◯ ◯◯ ◯◯ 15 2L 0.002 25 480 20 8 11.1 2.5 55 ◯◯ ◯◯ ◯◯ Exam- 16 2M0.002 25 560 5 6 9.2 2.1 25 ◯◯ ◯◯ ◯◯ ple 17 2N 0.002 10 520 8 9 10.0 1.98 Δ Δ ◯◯ Com- 18 2O 0.002 35 530 5 5 9.6 1.6 0 Δ Δ ◯◯ para- 19 2P 0.00215 490 12 6 10.0 1.9 7 Δ X ◯◯ tive 20 2Q 0.003 25 490 8 6 9.0 1.5 0 Δ Δ◯◯ Exam- 21 2R 0.002 25 490 7 7 8.2 1.6 0 Δ Δ ◯ ple 22 2S 0.002 25 50015 6 12.4 1.7 3 X X X 23 2T 0.002 25 480 18 7 11.2 1.8 8 X Δ ◯ 24 2U0.002 25 510 8 9 10.4 1.6 0 X X Δ 25 2V 0.002 25 500 9 8 9.9 1.7 6 Δ Δ◯◯ 26 2W 0.002 25 490 15 7 10.3 1.9 9 Δ ◯ Δ 27 2X 0.002 25 520 7 10 9.51.8 5 Δ Δ ◯◯ 28 2Y 0.002 25 480 15 8 10.5 1.4 0 X X Δ 29 2Z 0.002 25 47018 9 11.1 1.6 2 Δ Δ X 30 2B 0.010 35 480 12 7 9.5 1.9 5 Δ Δ Δ

From the evaluation results shown in Table 4, it is found thatgalvannealed steel sheets according to the invention (examples), incomparison with existing steel sheets (comparative examples), arelargely heightened in the interfacial strength between the coating layerand the base steel sheet and improved in the coating adhesion.

EXAMPLE 3

Each of steel ingots having a chemical composition shown in Table 5 washeated at 1250 degree centigrade to apply hot rolling followed byremoving a black skin on a surface, and thereby a hot rolled steel sheethaving a thickness of 2.0 mm was prepared. Subsequently, cold rolling atthe reduction rate of 65 percent was applied to form a cold rolled steelsheet having a thickness of 0.7 mm, followed by cutting out into a widthof 70 mm and a length of 180 mm, further followed by applying a primaryheating at 830 degree centigrade in a heating furnace in a nitrogenatmosphere that has a dew point of −30 degree centigrade and contains 3volume percent of hydrogen to apply surface cleaning, and thereby a basesteel sheet was obtained. The base steel sheet was dipped in 5 percenthydrochloric acid at 60 degree centigrade for 10 sec to apply pickling.Thereafter, recrystallization annealing and coating were applied by useof a laboratory galvanizing simulator. Conditions for therecrystallization annealing and the coating are as follows. TABLE 5 Thebalance of steel composition (mass %) is Fe Steel and inevitableimpurities No. C Si Mn P Others Note 3A 0.002 0.1 1.5 0.02 — Example 3B0.01 0.3 1 0.07 — 3C 0.007 0.1 2.2 0.05 — 3D 0.03 0.06 2 0.01 Cu: 0.2,Ni: 0.1 3E 0.002 0.5 1.5 0.07 — 3F 0.08 0.1 2 0.01 Cr: 0.05 3G 0.05 0.30.5 0.06 Mo: 0.15 3H 0.15 0.3 0.7 0.02 — 3I 0.1 0.25 2.6 0.06 Ca: 0.0053J 0.003 0.25 2 0.01 B: 0.001 3K 0.16 0.3 0.8 0.01 — 3L 0.25 0.5 2 0.012Mo: 0.3, B: 0.002, Ti: 0.02 3M 0.04 0.07 3 0.01 Sb: 0.01 3N 0.003 0.020.56 0.01 — Comparative 3O 0.003 0.04 0.34 0.065 B: 0.002 Example 3P0.003 0.03 0.5 0.04 — 3Q 0.002 0.02 0.5 0.04 — 3R 0.008 0.05 0.75 0.09 —3S 0.08 0.05 2 0.01 Cr: 0.05 3T 0.008 0.09 1 0.09 — 3U 0.004 0.02 0.140.021 — 3V 0.08 0.07 2 0.01 Ca: 0.005 3W 0.002 0.01 0.1 0.01 Mo: 0.15 3X0.01 0.02 0.45 0.01 — 3Y 0.12 0.02 1.5 0.015 — 3Z 0.08 0.06 1.5 0.03 Sb:0.01<Recrystallization annealing>

-   Atmosphere: 5 volume percent hydrogen+nitrogen (dew point: −35    degree centigrade)-   Temperature: 750 degree centigrade-   Holding time: 20 sec    <Coating Condition>-   Bath composition: Zn+0.14 mass percent Al (Fe saturation)-   Bath temperature: 460 degree centigrade-   Sheet temperature at the time of coating: 460 degree centigrade-   Coating time: 1 sec-   Concentration of oxygen in an atmosphere immediately before the    coating: conditions described in Table 6 (the balance 5 volume    percent hydrogen+nitrogen (dew point: −35 degree centigrade))

Obtained coating steel sheets contained 0.2 to 0.5 mass percent of Aland 0.5 to 2 mass percent of Fe in the coating layers. After the coatingprocess, the galvannealing process was applied in air in an electricheater. The temperature rise speeds and galvannealing temperatures inthe galvannealing process were the conditions described in Table 6.

Of each of obtained coating steel sheets, a cooling atmosphere from therecrystallization annealing to the coating, a thickness of a coatinglayer, a temperature rise speed, a temperature and a holding time in thegalvannealing process, a content of Fe in the coating layer, a ratio offine irregularity formed in an interface between the coating layer and abase steel sheet and a developed interfacial area ratio Sdr wereinvestigated similarly to a method explained in the example 1.Furthermore, in addition to the evaluation of the abovementioned coatingadhesion 1, evaluations of the coating adhesions 3 and 4 shown belowwere carried out. Results of these are shown in Table 6.

(Evaluation of the Coating Adhesion 3)

From each of the obtained steel sheets, a test piece having a width of40mm and a length of 100 mm was cut out followed by adhering a cellophanetape (width: 24 mm, manufactured by Nichiban Co., Ltd.) at a position ofa length 50 mm, a tape surface was bent inside by 90° followed byunbending, thereafter an amount of Zn adhered when the cellophane tapewas peeled was measured as the number of counts by means of X-rayfluorescence analysis. The number of measured counts of Zn wascompensated into the number of counts per unit length (1 m) of width oftest piece and evaluated according to the following criteria.

<Evaluation Criteria of the Coating Adhesion 3>

-   ◯◯: very good (number of counts: 500 or less)-   ◯: good (number of counts: more than 500 and 1000 or less)-   Δ: fair (number of counts: more than 1000 and 3000 or less)-   x: bad (number of counts: more than 3000)    (Evaluation of the coating adhesion 4)

From each of the obtained steel sheets, a test piece having a width of70 mm and a length of 150 mm was cut out, after dipping inrust-preventive oil 550 KH (manufactured by Nihon Parkerizing Co.,Ltd.), left in air for 24 hr while standing obliquely, and thus obtainedone was used as a test sample. A pressing test was carried out in whichin a state where both ends of a test sample 13 were clamped between adie 14 and a wrinkle suppressor 15 that form a bead die 16 such as shownin FIG. 9, from a back surface side of the test sample 13, a punch 17was pushed in to form a horseshoe shape. A surface of the die waspolished with #1000 polishing paper and accretions were cleansed everytime. A wrinkle suppressor force P was set at 12 kN and the punchingspeed was set at 100 mm/min. After the test, the test sample wasslightly degreased, followed by adhering a cellophane tape (width: 24mm, manufactured by Nichiban Corp.). An amount of Zn adhered to thecellophane tape when it was peeled was measured as the number of countsby X-ray fluorescence analysis, and evaluation was carried out accordingto the following criteria.

<Evaluation Criteria of the Coating Adhesion 4>

-   ◯◯: very good (number of counts: 50 or less)-   ◯: good (number of counts: more than 50 and 100 or less)-   Δ: fair (number of counts: more than 100 and 300 or less)

x: bad (number of counts: more than 300) TABLE 6 Concentration of oxygenin a cooling Galvannealed atmosphere until steel sheet the coating afterGalvannealing condition Coating layer Test the recrystallizationTemperature Galvannealing Holding Content sample Steel annealing risespeed temperature time Thickness of Fe No. No. (vol. %) (° C./s) (° C.)(s) (μm) (mass %) 1 3A 0.002 20 480 15 6 11.0 2 3A 0.002 25 490 10 310.5 3 3A 0.002 25 490 23 6 12.9 4 3A 0.001 30 520 25 14 11.0 5 3B 0.00125 490 10 7 9.2 6 3C 0.001 30 510 15 11 10.5 7 3D 0.002 25 490 10 9 10.28 3E 0.002 30 520 9 7 10.2 9 3F 0.003 25 490 15 9 11.5 10 3G 0.002 20470 25 6 10.9 11 3H 0.002 35 520 15 6 11.9 12 3I 0.002 20 460 10 4 8.913 3J 0.002 25 490 15 7 9.9 14 3K 0.002 20 460 30 7 10.5 15 3L 0.002 25480 20 6 10.8 16 3M 0.002 35 560 4 5 9.8 17 3N 0.002 20 520 20 12 12.518 3O 0.002 20 520 25 10 12.3 19 3P 0.002 20 490 15 6 11.5 20 3Q 0.00220 520 20 8 12.5 21 3R 0.002 20 490 25 8 11.2 22 3S 0.004 30 500 20 1012.5 23 3T 0.002 20 520 15 7 12.2 24 3U 0.002 30 510 8 7 10.2 25 3V0.002 30 480 15 8 9.8 26 3W 0.002 20 490 20 7 12.8 27 3X 0.001 20 480 1210 9.3 28 3Y 0.002 35 490 12 7 10.3 29 3Z 0.002 30 470 22 9 11.1 30 3D0.008 25 490 10 9 10.2 Galvannealed steel sheet Base steel Evaluationresult sheet Coating Developed Ratio of fine adhesion 1 Test interfacialirregularity in an Tensile sample area ratio Sdr interface Coating shearPeeling Coating No. (%) (%) adhesion 3 strength mode adhesion 2 Note 12.6 65 ◯◯ ◯◯ ◯◯ ◯◯ Example 2 2.3 30 ◯◯ ◯◯ ◯◯ ◯◯ 3 2.7 70 ◯ ◯◯ ◯◯ ◯ 4 2.660 ◯ ◯◯ ◯◯ ◯◯ 5 2.2 15 ◯◯ ◯◯ ◯◯ ◯◯ 6 2.4 40 ◯ ◯◯ ◯◯ ◯◯ 7 2.2 20 ◯◯ ◯◯ ◯◯◯◯ 8 2.5 50 ◯◯ ◯◯ ◯◯ ◯◯ 9 2.5 40 ◯ ◯◯ ◯◯ ◯◯ 10 2.6 60 ◯◯ ◯◯ ◯◯ ◯◯ 11 2.320 ◯ ◯◯ ◯◯ ◯◯ 12 2.1 15 ◯◯ ◯ ◯◯ ◯ 13 2.2 30 ◯◯ ◯◯ ◯◯ ◯◯ 14 2.1 20 ◯◯ ◯◯◯◯ ◯◯ 15 2.1 20 ◯◯ ◯◯ ◯◯ ◯◯ 16 2.1 20 ◯◯ ◯◯ ◯◯ ◯◯ 17 1.8 5 Δ Δ ◯ XComparative 18 1.9 5 Δ ◯ ◯ X Example 19 1.8 5 Δ Δ X ◯ 20 1.8 5 X Δ Δ Δ21 1.5 0 Δ X X Δ 22 1.5 0 X X X X 23 1.4 0 Δ X Δ ◯ 24 1.6 0 X X X Δ 251.5 0 ◯ Δ Δ ◯ 26 1.8 5 Δ ◯ ◯◯ Δ 27 1.5 0 ◯◯ Δ Δ ◯ 28 1.6 0 X X X Δ 291.6 0 X Δ Δ X 30 1.9 5 Δ Δ Δ Δ

From the evaluation results shown in Table 6, it is found thatgalvannealed steel sheets according to the invention (examples), incomparison with existing steel sheets (comparative examples), arelargely heightened in the interfacial strength between the coating layerand the base steel sheet and improved in the coating adhesion.

INDUSTRIAL APPLICABILITY

Since a galvannealed steel sheet according to the present invention is agalvannealed steel sheet that is remarkably excellent, in comparisonwith existing ones, in the coating adhesion at an interface between acoating layer and a base steel sheet, in the fields of automobiles, homeelectric appliances, construction materials and so on, there is noproblem of peeling of the coating layer at processing, appearance afterthe processing is excellent, and sufficient rust resistance can bemaintained. Accordingly, an industrially very useful effect in that thehigh mechanical strength and lighter weight can be attained for allshapes of components can be obtained.

1. A galvannealed steel sheet excellent in the coating adhesioncharacterized in that in an interface between a galvannealed layer and abase steel sheet on which the galvannealed layer is formed, anirregularity that has a depth of 10 nm or more at a pitch of 0.5 μm orless is present at least one per 5 μm of a length of the interface.
 2. Agalvannealed steel sheet excellent in the coating adhesion characterizedin that a surface shape of a base steel sheet that is observed after agalvannealed layer is peeled has a developed interfacial area ratio Sdrmeasured by use of a high-pass filter with a cut-off wavelength of 0.5μm of 2.0 percent or more.
 3. The galvannealed steel sheet excellent inthe coating adhesion according to claim 1 characterized in that the basesteel sheet contains, by mass percent, 0.25 percent or less of C, 0.03to 2.0 percent of Si and 0.005 to 0.07 percent of P and has acomposition satisfying the following equation (1). Note[C]+[P]≦[Si]  (1) Here, [C], [P] and [Si], respectively, mean contents(mass percent) of C, P and Si in the base steel sheet.
 4. Thegalvannealed steel sheet excellent in the coating adhesion according toclaim 3 characterized in that in a stage immediately before a coatinglayer is adhered to the base steel sheet, in order that Si contained inthe base steel sheet is not selectively oxidized on a surface, the basesteel sheet is heat treated before the coating layer is adhered.
 5. Thegalvannealed steel sheet excellent in the coating adhesion according toclaim 3 characterized in that in a base steel immediate below theinterface an oxide of silicon is contained.
 6. The galvannealed steelsheet excellent in the coating adhesion according to claim 3characterized in that the base steel sheet has a composition thatfurther includes, by mass percent, 5 percent or less of Mn, 0.01 percentor less of S and 0.08 percent or less of Al.
 7. The galvannealed steelsheet excellent in the coating adhesion according to claim 3characterized in that the base steel sheet has a composition thatfurther includes at least one kind selected from 0.2 percent or less ofTi, 0.2 percent or less of Nb and 0.2 percent or less of V, by masspercent.
 8. A method of manufacturing a galvannealed steel sheetexcellent in the coating adhesion characterized in that a base steelsheet that contains, by mass percent, 0.25 percent or less of C, 0.03 to2.0 percent of Si and 0.005 to 0.07 percent of P and has a compositionsatisfying the following equation (1) is heat treated so that Si in thesteel is not selectively surface oxidized, followed by cooling to acoating temperature in an atmosphere having an oxygen concentration of0.005 volume percent or less, further followed by dipping the base steelsheet in a molten zinc coating bath to form a coating layer, stillfurther followed by heating at a temperature rise speed of 20 degreecentigrade/s or more to a temperature range of 460 to 600 degreecentigrade and holding in the heating temperature range to apply agalvannealing process of the coating layer. Note[C]+[P]<[Si] (1) Here, [C], [P] and [Si], respectively, mean contents(mass percent) of C, P and Si in the base steel sheet.
 9. The method ofmanufacturing a galvannealed steel sheet excellent in the coatingadhesion according to claim 8 characterized in that the base steel sheethas a composition that further includes, by mass percent, 5 percent orless of Mn, 0.01 percent or less of S and 0.08 percent or less of Al.10. The method of manufacturing a galvannealed steel sheet excellent inthe coating adhesion according to claim 8 characterized in that the basesteel sheet has a composition that further includes at least one kindselected from 0.2 percent or less of Ti, 0.2 percent or less of Nb and0.2 percent or less of V, by mass percent and the temperature rise speedand a content of Si in the base steel sheet satisfy the followingequation (2). NoteST≧3.25/[Si]  (2) Here, in the equation, ST designates a temperaturerise speed (degree centigrade/s) and [Si] designates a content (masspercent) of Si in the steel sheet.
 11. The galvannealed steel sheetexcellent in the coating adhesion according to claim 2 characterized inthat the base steel sheet contains, by mass percent, 0.25 percent orless of C, 0.03 to 2.0 percent of Si and 0.005 to 0.07 percent of P andhas a composition satisfying the following equation (1). Note[C]+[P]≦[Si]  (1) Here, [C], [P] and [Si], respectively, mean contents(mass percent) of C, P and Si in the base steel sheet.
 12. Thegalvannealed steel sheet excellent in the coating adhesion according toclaim 11 characterized in that in a stage immediately before a coatinglayer is adhered to the base steel sheet, in order that Si contained inthe base steel sheet is not selectively oxidized on a surface, the basesteel sheet is heat treated before the coating layer is adhered.
 13. Thegalvannealed steel sheet excellent in the coating adhesion according toclaim 11 characterized in that in a base steel immediate below theinterface an oxide of silicon is contained.
 14. The galvannealed steelsheet excellent in the coating adhesion according to claim 11characterized in that the base steel sheet has a composition thatfurther includes, by mass percent, 5 percent or less of Mn, 0.01 percentor less of S and 0.08 percent or less of Al.
 15. The galvannealed steelsheet excellent in the coating adhesion according to claim 11characterized in that the base steel sheet has a composition thatfurther includes at least one kind selected from 0.2 percent or less ofTi, 0.2 percent or less of Nb and 0.2 percent or less of V, by masspercent.
 16. The method of manufacturing a galvannealed steel sheetexcellent in the coating adhesion according to claim 9 characterized inthat the base steel sheet has a composition that further includes atleast one kind selected from 0.2 percent or less of Ti, 0.2 percent orless of Nb and 0.2 percent or less of V, by mass percent and thetemperature rise speed and a content of Si in the base steel sheetsatisfy the following equation (2). NoteST>3.25/[Si]  (2) Here, in the equation, ST designates a temperaturerise speed (degree centigrade/s) and [Si] designates a content (masspercent) of Si in the steel sheet.